Production of sodium chlorate

ABSTRACT

An electrolytic cell comprising a tank having a closed top and an open bottom. An anode closure plate extends over the bottom of the tank to form the bottom wall of the cell. A plurality of vertically positioned anodes are secured to the closure plate and are spaced to receive a cathode member between adjacent pairs of anodes. The electrodes are mounted in the cell in a manner such that they are completely submerged in electrolyte in the cell.

United States Patent Goens et al. I

[451 July 11,1972

[541 PRODUCTION OF SODIUM CHLORATE [72] Inventors: Duane N. Goens,Fullerton; Thomas W.

Clapper, Whittier, both Of Calif.

[73] Assignee: Kerr McGee Chemicals Corp.

[22] Filed: Sept. 28, 1970 [21] Appl. No.: 76,306

Related US. Application Data [62] Division of Ser. No. 708,819, Feb. 28,1968, Pat. No.

52 U.S.Cl ..204/95,204/275,204/280,

- 204/284 51 1nt.Cl. ..C01b 11/26, BOlk 3 04 [58] Field ofSearch ..2o495 [56] References Cited UNITED STATES PATENTS 1,023,545 4/1912 Batesetal. ..204/95 1,547,362 7/1925 Casale ..204/27O OTHER PUBLlCATlONS Elec.Prod. of Bromates by Oserga, J. Electrochem Soc. Vol. 104 pp. 448- 451July 1957 Primary Examiner-F. C. Edmundson Atiorney-William G. AddisonABSTRACT An electrolytic cell comprising a tank having a closed top andan open bottom. An anode closure plate extends over the bottom of thetank to form the bottom wall of the cell. A plurality of verticallypositioned anodes are secured to the closure plate and are spaced toreceive a cathode member between adjacent pairs of anodes. Theelectrodes are mounted in the cell in a manner such that they arecompletely submerged in electrolyte in the cell.

4 Clains, 5 Drawing Figures l mini fllllllllllllllllllllllllllllllP'ATE'N'TEDJUL 11 m2 3, 67 6, 3 1 5 SHEET 1 OF 4 INVENTORS DUANE N.GOENS THOMAS W. CLAPPER PKTENTEDJUL 1 1 I972 SHEET 2 OF 4 FIG.2

PAYENIEmum m2 3,876,815

sum 3 or 4 FIG.3

INVENTORS DUANE N. GOENS THOMAS W. CLAPPER P'ATE'N'TEDJUL 1 1 m2 SHEET i0F 4 FIG.5

INVENTORS DUANE N. GOENS THOMAS W. CLAPPER PRODUCTION OF SODIUM CHLORATEThis application is a division of application Ser. No. 708,819 filedFeb. 28, 1968 now U.S. Pat. No. 3,598,715.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to an electrolytic cell which is capable of operating atrelatively high current efficiency over extended periods of time withlittle or no maintenance. The electrolytic cell of this invention isparticularly adapted for the electrochemical conversion of aqueoussodium chloride to sodium chlorate and, for convenience, the cell willbe described in connection with that process. It will be understood,however, that the cell of this invention is not limited in use to theproduction of sodium chlorate for it may be used in many otherelectrochemical operations, such as, for example, the manufacture ofalkali metal and alkaline earth metal hypochlorites, perchlorates,chlorine, caustic and the like.

2. Description of the Prior Art In general, sodium chlorate is producedcommercially by electrolysis of an aqueous solution of sodium chloridein an electrolytic cell. One type of sodium chlorate cell used todayemploys graphite anodes and steel cathodes which are partially submergedin an electrolyte in the cell. Generally, the anodes are in the form ofplates or rods and the cathodes are in the form of vertical sheets orcooling coils disposed in substantially parallel relationship betweenadjacent pairs of graphite anodes. In operation, hydrogen gas is evolvedat the cathode and elemental chlorine is generated at the anodes to formsodium hypochlorite in the electrolyte which is subsequently convertedto the sodium chlorate product.

It has been observed, however, that attrition of the graphite anodesoccurs during the electrolysis operation leading to an ever increasinganode-cathode gap with a corresponding increase in cell voltage and adecreasing efficiency of operation. In addition, when the electrodes areonly partially submerged in the electrolyte, corrosion of the electrodesoccurs at the interface of the electrolyte and the gases present in thecell, thereby reducing the useful life of the electrodes. Furthermore, aportion of the elemental chlorine formed at the anode may escape withthe cell gases making it necessary to add more acid to the electrolyteto maintain the pH of the electrolyte at a desired acid value.

SUMMARY OF THE INVENTION The present invention provides a compactelectrolytic cell which is capable of operating in a continuous manner,at high current efficiency, over extended periods of time. This cell hasa relatively low operating cost due, in part, to the prolonged life ofthe electrodes, which results in a substantial reduction in electrodematerial costs and cell rebuilding and maintenance costs.

Broadly, the electrolytic cell of this invention comprises a tank havinga closed top and an open bottom. An anode closure plate extends over thebottom of said tank and forms the bottom wall of the cell. A pluralityof vertically arranged, bot tom entering anodes and a plurality ofvertically disposed, substantially parallel cathode members are mountedwithin the cell. The anodes are secured to the closure plate and aresuitablyspaced to receive a cathode member between each adjacent pair ofanodes. Preferably, the anodes are formed of platinized titanium and maybe in the form of sheets, rods, strips or the like. The electrodes aremounted in the cell in a manner such that they are completely submergedin the electrolyte in the cell. This arrangement of the electrodespermits substantially all of the platinum surface to be effectively usedas anode surface and prevents corrosion of the electrodes at theinterface of the electrolyte and the cell gases, as frequently occurs inelectrolytic cells used heretofore.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a view, partly in elevation andpartly in section of an embodiment of the electrolytic cell of thisinvention.

FIG. 2 is a view, partly in elevation and partly in section of anotherembodiment of the cell.

FIG. 3 is a top plan view of a portion of the electrolytic cell shown inFIG. 2 with a portion broken away to show the novel anode-cathodearrangement of this embodiment.

FIG. 4 is a sectional view of a portion of the top of the cellillustrating an embodiment for introducing electrolyte into the cell.

FIG. 5 is an exploded view of the electrolytic cell of this inventionshowing the relationship of the components of the cell.

DESCRIPTION OF THE EMBODIMENTS Referring to FIG. 1 of the drawing thereis illustrated the electrolytic cell 10 of this invention whichcomprises a tank 12 having a closed top 14 and an open bottom. The celltop 14 may be integral with the tank or may be removably secured theretoby suitable means, such as bolts 15. Preferably, a removable top isprovided in order to facilitate assembly of the cell and alignment ofthe electrodes. The upper portion of the tank walls may be flaredoutwardly to form flanges 16 which provide support and mounting meansfor the removable top. The bottom edges of the tank wall may also beflared outwardly to form flanges 17 which provide support and mountingmeans for anode closure plate 20 (consisting of layers 21 and 22) whichextends over the bottom of the tank and forms the bottom wall of thecell. Plate 20 is removably secured to the tank by suitable means suchas bolts 19.

The tank 12 preferably is formed of an electrically conductive metal,with current being conducted to the walls of the tank. The entire tankis cathodically protected against corrosion, thereby enabling the tankto be constructed of iron or mild steel. However, the tank, if desired,may also be formed of an electrically non-conductive material such asceramics, rubber, plastics and the like. When the tank is formed of aconductive metal, an electrically insulating gasket 28 is providedbetween flanges 17 and anode closure plate 20 to insulate the plate fromthe tank, and bolts 19 are insulated from flanges 17 or closure plate 20by suitable means (not shown). Insulating base members 23 are preferablyare provided to support the cell.

The cell top 14 is also formed of a corrosion resistant material whichmay be the same as, or different from, the material of which the tank 12is formed. Preferably, the top is formed of an electricallynon-conductive material such as polyvinyl chloride, polyvinyldichloride, polyvinyl difluoride, natural or synthetic rubber, polyesterresins, phenolic resins and the like.

The cell top is provided with an opening 24 through which gases areremoved from the cell. The electrolyte may also be introduced into thecell through opening 24. Alternatively, a separate opening 25 may beprovided in the cover to introduce electrolyte into the cell. Effluentcell liquor is removed through outlet 26.

The cell is provided with means which substantially prevents freshelectrolyte introduced into the cell from short circuiting" to theoutlet. According to one embodiment of the invention, distributing meansmay be provided within the cell for distributing the electrolyteuniformly therein. Thus, such means may comprise a perforated conduit(not shown) extending inside the cell from opening 25 along the bottomwall of the cell through which electrolyte is uniformly distributedacross the bottom of the cell.

According to another embodiment, as shown in FIG, 4, packing material 30is disposed within opening 24 in the cell top and is held in place byany suitable corrosion resistant supporting means 31 such as screening,mesh, perforated plates or the like. The packing material 30 may consistof saddles, beads, pellets, fibers, or the like of a material, such astetrafuloroethylene fluocarbon resins, which is inert to the electrolyteand the products of electrolysis. Electrolyte is conveyed to the cell byconduit means 32 and is introduced into the cell through packing 30. Gasformed in the cell during electrolysis passes upwardly through packing30 and is removed through conduit means 33. Thus, fresh electrolytepasses through packing 30 countercurrent to the exiting cell gaseswhich, in the production of sodium chlorate, contain hydrogen andchlorine gases. The introduction of the electrolyte in this manner notonly facilitates uniform distribution of electrolyte in the cell butprovides the additional advantage of scrubbing chlorine from the exitingcell gases. Thus, since the electrolyte introduced into the cell has arelatively low chlorine content, substantially all of the chlorine isscrubbed from the gaseous cell effluent when it is contacted with theelectrolyte in this manner. Consequently, the gas stream recovered fromthe cell is substantially pure hydrogen and may be recovered. Inaddition, the return of the chlorine to the cell in this manner tends toreduce the amount of acid required to be added to the electrolyte tomaintain the pH of the cell liquor at a desired acid value.

The anode closure plate 20 which forms the bottom wall of the cell has aplurality of anode members 35 secured thereto in spaced apart,substantially parallel relationship to each other. Anode members 35 aremounted perpendicularly on plate 20, see FIG 5; and are secured theretoby any suitable means such as by welding or the like. According to apreferred embodiment of the invention, the anode members consist of anelectrically conductive base metal provided with a thin surface layer ofa noble metal or noble metal alloy. Any of the valve metals, namelytitanium, tantalum, zirconium, aluminum, niobium and alloys thereof maybe used as the base metal. The noble metal may be selected from thegroup consisting of platinum, iridium, rhodium, palladium, ruthenium andalloys thereof. Generally, it is preferred to use anodes consisting oftitanium metal or a titanium alloy as the base metal coated withplatinum metal or platinum alloy. The noble metal may be applied to thebase in any suitable manner such as by roll cladding, explosivecladding, thermal decomposition, electrolyte decomposition and the like.A layer of noble metal or noble metal alloy of between about 5 to 500microinches, and preferably between about 20 to 100 micro-inches, inthickness is provided on the base metal. The thickness of the base metalportion of the anodes is such that the anodes are sufficiently rigid torequire no additional support when secured to the closure plate, asdescribed above. Commercially available sheets 0. l to 0.15 inch inthickness are particularly suitable. However, sheets as thin as 0.05 oras thick as 0.5 inch may be used. Although the use of anodes formed of aconductive base metal surfaced with a noble metal is generally preferredin the cell of this invention, it is to be understood that graphiteanodes may also be used in the cell if desired.

While the anode members 35 preferably are in the form of sheets, asillustrated in FIG. 3, other suitable configurations, such as strips,rods, metal mesh and the like, may also be used. The structure of thecell insures that the anode members are completely submerged inelectrolyte during operation of the cell. Thus, the members extendvertically from the anode closure plate to a point below the level ofthe electrolyte in the tank.

As shown in FIG. 1, anode closure plate 20 consists of two metal layersintegrally bonded together. The upper surface 21 of the plate, to whichthe anode members 35 are secured, is formed of a metal selected from thegroup consisting of titanium, tantalum and columbium or other metalwhich is resistant to the corrosive action of the electrolyte in thecell. Due to the relatively poor electrical conductivityof these metalsa lower or backing layer 22 of a more conductive metal, such as copper,aluminum, mild steel or the like is secured to upper layer 21 tofacilitate uniform distribution of current to the anodes. Anode bus bars36 are affixed to the conductive backing layer.

A plurality of cathode members are provided in the cell, with a cathodemember being provided between each adjacent pair of anode members. Thecathode members are mounted in the cell so that they are completelysubmerged in electrolyte.

The cathode members may be of any suitable structural design capable ofbeing fitted between the parallel anode members. As shown in FIG. 1, thecathode members may consist of vertically disposed conductive metalsheets 40. The sheets 40 may be supported in the cell by any suitablemeans such as by integrally attaching the sheets to the inside surfaceof opposing walls of the tank. The sheets 40 are suitably spaced toreceive an anode member 35 between each adjacent pair of cathode sheets.Since the sheets 40 are covered by electrolyte they may be formed ofiron or mild steel. The thickness of such sheets may vary from about 0.1to 0.5 inch.

Preferably, the electrolytic cell of this invention utilizes as thecathode members, the cathode assembly disclosed and claimed in patentapplication Ser. No. 708,820, entitled CATI-IODE ASSEMBLY," by Duane N.Goens and George G. Gale, filed Feb. 28, 1968, and assigned to the sameassignee as the present application. As shown in FIGS. 2, 3 and 5 aplurality of such cathode assemblies 42 are secured, as by welding orthe like, to the inside surface of opposing sidewalls of the tank withthe cathode assemblies 42 being spaced to receive an anode memberbetween each adjacent cathode assembly. In this manner, each anodesurface opposes a cathode surface so that all of the anode surfaces areprotected from erosion by the gas evolved at the cathode surfaces. Thecathode assemblies 42 and the anode members 35 are positioned in thecell as close together as possible without contacting one another, inorder to minimize IR drop in the cell and thereby maintain high powerefficiency. Particularly satisfactory results are obtained when thespacing between the anode and cathode surfaces is about 0.25 inch.

According to this embodiment, each cathode assembly comprises a pair ofopposing cathodes having passage means therebetween. Each cathode of theassembly includes means for directing gas evolved at such cathode duringelectrolysis into the passage means between the opposing cathodeswhereby the gas is directed from its natural upward path between theelectrodes into the passage means between the opposing cathodes and ismaintained within the cathode assembly as it bubbles to the surface ofthe electrolyte. Preferably, as illustrated in FIGS. 2, 3 and 5, eachcathode assembly 42 is formed of a pair of substantially parallel sheetsof expanded metal 43, with each sheet 43 forming a cathode of theassembly. The cathodes are maintained in spaced apart relationship toprovide a continuous passage means 46 between the cathodes. The sheetsmay be maintained in this position by any suitable means, such as forexample, spacing bars 45 vertically positioned between the sheets.Generally, the expanded metal is a continuous fabric of an open meshnetwork of interconnected, outwardly extending webs 44 which encloseopenings of diamond, oval or other shapes. The webs 44 are, in general,flat in cross-section and are positioned at an angle of between about 20and preferably between about 35 and 55 to the plane of the originalsheet. If desired, only a single sheet of expanded metal may be disposedbetween each endwall of the tank and the anode member adjacent saidendwall.

The electrolytic cell of this invention is well adapted for use in acontinuous electrochemical process wherein an electrolyte is introducedinto the cell into contact with the electrodes until the reaction hasproceeded to a desired degree, whereupon the electrolyte is removed fromthe cell, passed to an external holding tank and recovered orrecirculated to the same cell or an adjacent cell for furtherelectrochemical treatment, if desired. The present cell may also be usedin a batch process operation in which the electrolyte is circulated inthe cell until the desired electrochemical reaction is completed.

One of the uses for which the electrolytic cell of this invention isparticularly well suited is a continuous process for the production ofchlorine dioxide. In such a process, an aqueous solution containingsodium chloride is electrolyzed in the cell of the present invention toprovide a cell liquor rich in sodium chlorate. That liquor is removedfrom the cell and passed to a reactor where it is contacted with HCl toform a gaseous mixture of chlorine dioxide and chlorine gas, which isrecovered, and a solution which is depleted of chlorate values. Thissolution is then recycled to the electrolytic cell where it is againsubjected to electrolysis to increase the concentration of sodiumchlorate. This cycle may be continuously repeated. In this manner,substantially all of the sodium chlorate produced in the cell isconverted into G and Cl with a high degree of efficiency.

The temperature of the electrolyte in the cell of this inventionpreferably is controlled by the introduction and circulation of cooledelectrolyte into the cell.

The electrolytic cell of the present invention may be operated at highcurrent densities with low voltage and with high current efficiency.Thus, the cell may be operated at a current density of between about 25to 1,000 amperes or more per square foot, so that a small compact cellis capable of a high production rate. The cell, when used in theelectrolysis of aqueous sodium chloride solution to sodium chlorate, maybe operated at a cell voltage of from about 3.0 to 4.0 volts with acurrent efficiency of 95 to 100 percent.

The present cell, when containing noble metal surfaced anodes, iscapable of being operated at a substantially higher current density inthe electrolysis of sodium chloride to sodium chlorate than conventionalgraphite-anode chlorate cells and may be operated at temperatures higherthan generally used in chlorate production. The noble metal surfacedanodes are resistant to oxidation and corrosion even at temperatures of100 C. or above. Whereas graphite anodes tend to wear away in use,leading to an ever increasing anode-cathode gap with a correspondingincrease in cell voltage, the noble metal surfaced anodes are relativelypermanent in nature so that the original spacing between the electrodesmay be maintained for long periods of time. Thus, high conductivity ismaintained and the cell retains its original high efficiency.

When the novel cathode assembly configuration shown in FIGS. 2, 3 and 5is used in the present cell, its use insures that cathode gases aredirected away from the anode surfaces thereby preventing the gas fromeroding away the noble metal surface and minimizing the amount of gaspresent between the electrodes. In this manner, a low IR drop in thecell is maintained. Because of the high, constant current density, therelatively small compact cell of this invention has a relatively lowoperating cost due to the prolonged life of the electrodes which resultsin a substantial reduction in cell rebuilding and maintenance costs.

EXAMPLE The following example illustrates the use of the electrolyticcell of this invention in the electrochemical conversion of an aqueoussolution of sodium chloride to sodium chlorate. An electrolytic cell wasdesigned and constructed as shown in F IG. 5 and contained the cathodeassemblies shown in FIGS. 2, 3 and 5. The cell contained 20 anodeswelded to a closure plate formed of a thin titanium sheet bonded to amild steel plate. The anodes were formed of platinized titanium sheets,each about 0.1 inch in thickness and having from 25 to 50 micro-inchesof platinum coated on each side of the titanium sheet. Twenty-onecathode assemblies, each constructed of two sheets of expanded mildsteel separated by /2 inch steel vertical spacing bars were welded tothe inside walls of a mild steel tank. The electrodes were mounted inthe cell to provide a A inch spacing between adjacent cathode and anodesurfaces.

An aqueous electrolyte containing about 265 grams per liter sodiumchlorate, about 170 grams per liter sodium chloride and about 3 gramsper liter sodium dichromate was introduced into the cell through anopening 25 in the cell cover. Sufficient electrolyte was introduced intothe cell to completely cover the electrodes. Another portion of theelectrolyte was introduced into an external holding tank from which theelectrolyte was circulated through the cell during electrolysis. Thecell was operated under the following conditions:

Current density 150 amps/ sq. ft. Cell voltage 3.04-3.20 voltsTemperature 60 C.

pl-l A 6.97.2

During operation of the cell, electrolyte was continuously circulatedbetween the cell and the external holding tank, with the electrolytebeing removed from the cell through outlet 26 and being reintroducedthrough opening 25. Cell operating temperature was controlled by coolingthe electrolyte passing from the holding tank to the cell and byadjusting the electrolyte feed rate.

Substantially all of the hydrogen gas evolved at the cathodes duringelectrolysis was directed into the space between the cathode surfacesand was retained between the cathode surfaces as it'bubbled to thesurface of the electrolyte and was removed from the cell through opening24.

The cell was operated continuously for 10 days. At the end oftheelectrolysis operation the sodium chloride concentration in theelectrolyte had been reduced to about 1 10 grams per liter while thesodium chlorate concentration had increasedto about 380 grams per liter.Current efficiency of the cell during electrolysis was about 96 97percent, based on the formation of sodium chlorate. Platinum loss ratefrom the anodes was very low, 0.012 Troy ounces per ton of chlorateformed.

While the invention has been described in connection with specificembodiments thereof, it will be understood that it is capable of furthermodification and this application is intended to cover any variations,uses or adaptations of the invention. It will therefore be recognizedthat the invention is not to be considered as limited to the preciseembodiments shown and described but is to be interpreted as broadly aspermitted by the appended claims.

What is claimed is:

1. A process for the electrochemical conversion of sodium chloride in anaqueous solution to sodium chlorate which comprises introducing saidaqueous solution into an electrolytic cell which contains a plurality ofanode members and a plurality of cathode assemblies, each of saidcathode assemblies comprising a pair of opposed, inclined surfaceshaving a passage therebetween,

passing an electrical current through said anode members and saidcathode assemblies to evolve hydrogen gas at said cathodes and generatechlorine at said anodes, a major portion of said chlorine reacting toform sodium chlorate in the electrolyte and a minor portion of saidchlorine being evolved as chlorine gas,

directing said hydrogen gas inwardly and upwardly between said pair ofinclined surfaces and away from said anodes and removing said hydrogengas said chlorine gas and said electrolyte from the cell.

2. The process defined in claim 1 in which said aqueous solution isintroduced into said cell countercurrent to and in contact with saidgases being removed from the cell whereby at least a portion of thechlorine gas is absorbed in said aqueous solution.

3. The process as defined in claim 1 in which each of said anode memberscomprise a sheet of a base metal having a noble metal surface.

4. The process as defined in claim 1 in which said cell is operated at acurrent density of about 100 to 300 amperes per square foot and avoltage of between 3 to 4 volts until a substantial amount of saidsodium chloride is converted to sodium chlorate.

2. The process defined in claim 1 in which said aqueous solution isintroduced into said cell countercurrent to and in contact with saidgases being removed from the cell whereby at least a portion of thechlorine gas is absorbed in said aqueous solution.
 3. The process asdefined in claim 1 in which each of said anode members comprise a sheetof a base metal having a noble metal surface.
 4. The process as definedin claim 1 in which said cell is operated at a current density of about100 to 300 amperes per square foot and a voltage of between 3 to 4 voltsuntil a substantial amount of said sodium chloride is converted tosodium chlorate.